int i;
int j;

void setup() {
  for (j = 1; j <= 10; j++) {
    pinMode(j, OUTPUT);
  }
}

void loop() {
  for (i = 1; i <= 10; i++)  {
    digitalWrite(i, HIGH);
    delay(50);
    digitalWrite(i, LOW);
    delay(50);
  }
  for (i = 9; i > 1; i--)  {
    digitalWrite(i, HIGH);
    delay(50);
    digitalWrite(i, LOW);
    delay(50);
  }
}